Thermal conductive sheet

ABSTRACT

A thermal conductive sheet contains a plate-like boron nitride particle. The thermal conductive sheet has a thermal conductivity in a direction perpendicular to the thickness direction of the thermal conductive sheet of 4 W/m·K or more. The thermal conductive sheet does not fall off from an adherend in the initial adhesion test ( 1 ) below: 
     Initial adhesion test ( 1 ): 
     The thermal conductive sheet is temporarily fixed on an adherend along the horizontal direction, and thereafter the adherend is turned over to be upside down. The maximum cutting resistance on the cutting blade at the time of cutting the thermal conductive sheet is 120 N/30 mm or less.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese PatentApplications No. 2010-018256 filed on Jan. 29, 2010; No. 2010-090908filed on Apr. 9, 2010; No. 2010-161850 filed on Jul. 16, 2010; and No.2010-161854 filed on Jul. 16, 2010, the contents of which are herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal conductive sheet, to bespecific, to a thermal conductive sheet for use in power electronicstechnology.

2. Description of Related Art

In recent years, power electronics technology which uses semiconductorelements to convert and control electric power is applied in hybriddevices, high-brightness LED devices, and electromagnetic inductionheating devices. In power electronics technology, a high current isconverted to, for example, heat, and therefore materials that aredisposed near the semiconductor element are required to have excellentheat dissipation characteristics (excellent heat conductivity).

For example, Japanese Unexamined Patent Publication No. 2010-10469 hasproposed covering a CPU with silicone resin, and then providing aheatsink so as to cover them.

Also, for example, a thermal conductive sheet containing a plate-likeboron nitride powder and an acrylic ester copolymer resin has beenproposed (e.g., see Japanese Unexamined Patent Publication No.2008-280496).

In the thermal conductive sheet of Japanese Unexamined PatentPublication No. 2008-280496, the boron nitride powder is oriented so asto orient its major axis direction (direction perpendicular to the platethickness of the boron nitride powder) in the thickness direction of thesheet, and thermal conductivity in the thickness direction of thethermal conductive sheet is improved in this way.

SUMMARY OF THE INVENTION

However, the silicone adhesive of Japanese Unexamined Patent PublicationNo. 2010-10469 is disadvantageous in that heat generated from the CPUcannot be conducted to the heatsink efficiently.

Another disadvantage of the silicone resin of Japanese Unexamined PatentPublication No. 2010-10469 is that when the silicone resin is applied tothe CPU, positioning in relation to the CPU easily fails, failing toreliably fix the silicone resin to the CPU.

However, there are cases where the thermal conductive sheet is requiredto have a high thermal conductivity in a direction (plane direction)perpendicular to the thickness direction depending on its use andpurpose. In such a case, the thermal conductive sheet of Patent Document1 is disadvantageous in that the major axis direction of the boronnitride powder is perpendicular to (crossing) the plane direction, andtherefore the thermal conductivity in the plane direction isinsufficient.

Furthermore, such a thermal conductive sheet is usually processed into apredetermined shape by die cutting and the like, and therefore thethermal conductive sheet is required to have low brittleness, andrequired that no chipping or fracture is caused in the sheet at the timeof die cutting.

An object of the present invention is to provide a thermal conductivesheet that is excellent in thermal conductivity in the plane direction,and also excellent in initial adhesion.

Another object of the present invention is to provide a thermalconductive sheet that is excellent in thermal conductivity in the planedirection, and also capable of achieving low brittleness.

A thermal conductive sheet of the present invention contains aplate-like boron nitride particle, wherein the thermal conductivity in adirection perpendicular to the thickness direction of the thermalconductive sheet is 4 W/m·K or more, and the thermal conductive sheetdoes not fall off from an adherend in the initial adhesion test (1)below.

Initial adhesion test (1): The thermal conductive sheet is temporarilyfixed on an adherend along the horizontal direction, and thereafter theadherend is turned over to be upside down.

The thermal conductive sheet of the present invention is excellent inthermal conductivity in the plane direction that is perpendicular to thethickness direction, and also excellent in adhesion (initial adhesion)to an adherend after being thermocompression bonded at a predeterminedtemperature.

Therefore, by thermocompression bonding the thermal conductive sheet ofthe present invention to an adherend at a predetermined temperature, thethermal conductive sheet can be reliably fixed (temporarily fixed) onthe adherend.

Thus, the thermal conductive sheet allows the heat from the adherend tobe reliably dissipated.

The thermal conductive sheet of the present invention contains aplate-like boron nitride particle, wherein the thermal conductivity in adirection perpendicular to the thickness direction of the thermalconductive sheet is 4 W/m·K or more, and the maximum cutting resistanceon the cutting blade is 120 N/30 mm or less at the time of cutting thethermal conductive sheet.

The thermal conductive sheet of the present invention is excellent inthermal conductivity in the plane direction that is perpendicular to thethickness direction, and furthermore, the thermal conductive sheet ofthe present invention has a low maximum cutting resistance at the timeof cutting, that is, brittleness is suppressed.

Therefore, the thermal conductive sheet of the present invention can beapplied in various heat dissipation uses as a thermal conductive sheetthat can be easily processed, that is excellent in handleability, andthat is excellent in thermal conductivity in the plane direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of an embodiment of a thermal conductivesheet of the present invention.

FIG. 2 shows process drawings for describing a method for producing thethermal conductive sheet shown in FIG. 1:

(a) illustrating a step of hot pressing a mixture or a laminated sheet,

(b) illustrating a step of dividing the pressed sheet into a pluralityof pieces, and

(c) illustrating a step of laminating the divided sheets.

FIG. 3 shows a perspective view of a test device (Type I, before bendtest) of a bend test.

FIG. 4 shows a perspective view of a test device (Type I, during bendtest) of a bend test.

DETAILED DESCRIPTION OF THE INVENTION

A thermal conductive sheet of the present invention contains boronnitride particles.

To be specific, the thermal conductive sheet contains boron nitride (BN)particles as an essential component, and further contains, for example,a resin component.

The boron nitride particles are formed into a plate-like (or flake-like)shape, and are dispersed so as to be orientated in a predetermineddirection (described later) in the thermal conductive sheet.

The boron nitride particles have an average length in the longitudinaldirection (maximum length in the direction perpendicular to the platethickness direction) of, for example, 1 to 100 μm, or preferably 3 to 90μm. The boron nitride particles have an average length in thelongitudinal direction of, 5 μm or more, preferably 10 μm or more, morepreferably 20 μm or more, even more preferably 30 μm or more, or mostpreferably 40 μm or more, and usually has an average length in thelongitudinal direction of, for example, 100 μm or less, or preferably 90μm or less.

The average thickness (the length in the thickness direction of theplate, that is, the length in the short-side direction of the particles)of the boron nitride particles is, for example, 0.01 to 20 μm , orpreferably 0.1 to 15 μm.

The aspect ratio (length in the longitudinal direction/thickness) of theboron nitride particles is, for example, 2 to 10000, or preferably 10 to5000.

The average particle size of the boron nitride particles as measured bya light scattering method is, for example, 5 μm or more, preferably 10μm or more, more preferably 20 μm or more, particularly preferably 30 μmor more, or most preferably 40 μm or more, and usually is 100 μm orless.

The average particle size as measured by the light scattering method isa volume average particle size measured with a dynamic light scatteringtype particle size distribution analyzer.

When the average particle size of the boron nitride particles asmeasured by the light scattering method is below the above-describedrange, the thermal conductive sheet may become fragile, andhandleability may be reduced.

The bulk density (JIS K 5101, apparent density) of the boron nitrideparticles is, for example, 0.3 to 1.5 g/cm³, or preferably 0.5 to 1.0g/cm³.

As the boron nitride particles, a commercially available product orprocessed goods thereof can be used. Examples of commercially availableproducts of the boron nitride particles include the “PT” series (forexample, “PT-110”) manufactured by Momentive Performance Materials Inc.,and the “SHOBN®UH1” series (for example, “SHOBN®UHP-1”) manufactured byShowa Denko K.K.

The resin component is a component that is capable of dispersing theboron nitride particles, i.e., a dispersion medium (matrix) in which theboron nitride particles are dispersed, including, for example, resincomponents such as a thermosetting resin component and a thermoplasticresin component.

Examples of the thermosetting resin component include epoxy resin,thermosetting polyimide, phenol resin, urea resin, melamine resin,unsaturated polyester resin, diallyl phthalate resin, silicone resin,and thermosetting urethane resin.

Examples of the thermoplastic resin component include polyolefin (forexample, polyethylene, polypropylene, and ethylene-propylene copolymer),acrylic resin (for example, polymethyl methacrylate), polyvinyl acetate,ethylene-vinyl acetate copolymer, polyvinyl chloride, polystyrene,polyacrylonitrile, polyamide, polycarbonate, polyacetal, polyethyleneterephthalate, polyphenylene oxide, polyphenylene sulfide, polysulfone,polyether sulfone, poly ether ether ketone, polyallylsulfone,thermoplastic polyimide, thermoplastic urethane resin,polyamino-bismaleimide, polyamide-imide, polyether-imide,bismaleimide-triazine resin, polymethylpentene, fluorine resin, liquidcrystal polymer, olefin-vinyl alcohol copolymer, ionomer, polyarylate,acrylonitrile-ethylene-styrene copolymer,acrylonitrile-butadiene-styrene copolymer, and acrylonitrile-styrenecopolymer,

These resin components can be used alone or in combination of two ormore.

Of the resin components, preferably, the epoxy resin is used.

The epoxy resin is in a state of liquid, semi-solid, or solid undernormal temperature.

To be specific, examples of the epoxy resin include aromatic epoxyresins such as bisphenol epoxy resin (e.g., bisphenol A epoxy resin,bisphenol F epoxy resin, bisphenol S epoxy resin, hydrogenated bisphenolA epoxy resin, dimer acid-modified bisphenol epoxy resin, and the like),novolak epoxy resin (e.g., phenol novolak epoxy resin, cresol novolakepoxy resin, biphenyl epoxy resin, and the like), naphthalene epoxyresin, fluorene epoxy resin (e.g., bisaryl fluorene epoxy resin and thelike), and triphenylmethane epoxy resin (e.g., trishydroxyphenylmethaneepoxy resin and the like); nitrogen-containing-cyclic epoxy resins suchas triepoxypropyl isocyanurate (triglycidyl isocyanurate) and hydantoinepoxy resin; aliphatic epoxy resin; alicyclic epoxy resin (e.g., dicycloring-type epoxy resin and the like); glycidylether epoxy resin; andglycidylamine epoxy resin.

These epoxy resins can be used alone or in combination of two or more.

Preferably, a combination of a liquid epoxy resin and a solid epoxyresin is used, or more preferably, a combination of a liquid aromaticepoxy resin and a solid aromatic epoxy resin is used. Examples of such acombination include, to be more specific, a combination of a liquidbisphenol epoxy resin and a solid triphenylmethane epoxy resin, and acombination of a liquid bisphenol epoxy resin and a solid bisphenolepoxy resin.

Preferably, a semi-solid epoxy resin is used alone, or more preferably,a semi-solid aromatic epoxy resin is used alone. Examples of those epoxyresins include, in particular, a semi-solid fluorene epoxy resin.

A combination of a liquid epoxy resin and a solid epoxy resin, or asemi-solid epoxy resin can improve conformability to irregularities(described later) of the thermal conductive sheet.

The epoxy resin has an epoxy equivalent of, for example, 100 to 1000g/eqiv., or preferably 180 to 700 g/eqiv., and has a softeningtemperature (ring and ball test) of, for example, 80° C. or less (to bespecific, 20 to 80° C.), or preferably 70° C. or less (to be specific,25 to 70° C.).

The epoxy resin has a melt viscosity at 80° C. of, for example, 10 to20,000 mPa·s, or preferably 50 to 15,000 mPa·s. When two or more epoxyresins are used in combination, the melt viscosity of the mixture ofthese epoxy resins is set within the above-described range.

Furthermore, when an epoxy resin that is solid under normal temperatureand an epoxy resin that is liquid under normal temperature are used incombination, a first epoxy resin having a softening temperature of, forexample, below 45° C., or preferably 35° C. or less, and a second epoxyresin having a softening temperature of, for example, 45° C. or more, orpreferably 55° C. or more are used in combination. In this way, thekinetic viscosity (in conformity with JIS K 7233, described later) ofthe resin component (mixture) can be set to a desired range, and also,conformability to irregularities of the thermal conductive sheet can beimproved.

The epoxy resin can also be prepared as an epoxy resin compositioncontaining, for example, an epoxy resin, a curing agent, and a curingaccelerator.

The curing agent is a latent curing agent (epoxy resin curing agent)that can cure the epoxy resin by heating, and examples thereof includean imidazole compound, an amine compound, an acid anhydride compound, anamide compound, a hydrazide compound, and an imidazoline compound. Inaddition to the above-described compounds, a phenol compound, a ureacompound, and a polysulfide compound can also he used.

Examples of the imidazole compound include 2-phenyl imidazole, 2-methylimidazole, 2-ethyl-4-methyl imidazole, and2-phenyl-4-methyl-5-hydroxymethyl imidazole.

Examples of the amine compound include aliphatic polyamines such asethylene diamine, propylene diamine, diethylene triamine, andtriethylene tetramine; and aromatic polyamines such as methaphenylenediamine, diaminodiphenyl methane, and diaminodiphenyl sulfone.

Examples of the acid anhydride compound include phthalic anhydride,maleic anhydride, tetrahydrophthalic anhydride, hexahydrophthalicanhydride, 4-methyl-hexahydrophthalic anhydride, methyl nadic anhydride,pyromelletic anhydride, dodecenylsuccinic anhydride, dichloro succinicanhydride, benzophenone tetracarboxylic anhydride, and chlorendicanhydride.

Examples of the amide compound include dicyanodiamide and polyamide.

An example of the hydrazide compound includes adipic acid dihydrazide.

Examples of the imidazoline compound include methylimidazoline,2-ethyl-4-methylimidazoline, ethylimidazoline, isopropylimidazoline,2,4-dimethylimidazoline, phenylimidazoline, undecylimidazoline,heptadecylimidazoline, and 2-phenyl-4-methylimidazoline.

These curing agents can be used alone or in combination of two or more.

A preferable example of the curing agent is an imidazole compound.

Examples of the curing accelerator include tertiary amine compounds suchas triethylenediamine and tri-2,4,6-dimethylaminomethylphenol;phosphorus compounds such as triphenylphosphine, tetraphenylphosphoniumtetraphenylborate, andtetra-n-butylphosphonium-o,o-diethylphosphorodithioate; a quaternaryammonium salt compound; an organic metal salt compound; and derivativesthereof. These curing accelerators can be used alone or in combinationof two or more.

In the epoxy resin composition, the mixing ratio of the curing agent is,for example, 0.5 to 50 parts by mass, or preferably 1 to 10 parts bymass per 100 parts by mass of the epoxy resin, and the mixing ratio ofthe curing accelerator is, for example, 0.1 to 10 parts by mass, orpreferably 0.2 to 5 parts by mass per 100 parts by mass of the epoxyresin.

The above-described curing agent, and/or the curing accelerator can beprepared and used, as necessary, as a solution, i.e., the curing agentand/or the curing accelerator dissolved in a solvent; and/or as adispersion liquid, i.e., the curing agent and/or the curing acceleratordispersed in a solvent.

Examples of the solvent include organic solvents including ketones suchas acetone and methyl ethyl ketone, ester such as ethyl acetate, andamide such as N,N-dimethylformamide; and water-based solvents includingwater, and alcohols such as methanol, ethanol, propanol, andisopropanol. A preferable example is an organic solvent, more preferableexamples are ketones and amides.

Of the thermoplastic resin components, preferably, polyolefin is used.

Preferable examples of polyolefin are polyethylene andethylene-propylene copolymer.

Examples of polyethylene include a low density polyethylene and a highdensity polyethylene.

Examples of ethylene-propylene copolymer include a random copolymer, ablock copolymer, or a graft copolymer of ethylene and propylene.

These polyolefins can be used alone or in combination of two or more.

The polyolefins have a weight average molecular weight and/or a numberaverage molecular weight of, for example, 1000 to 10000.

The polyolefin can be used alone, or can be used in combination.

The resin component has a kinetic viscosity as measured in conformitywith the kinetic viscosity test of JIS K 7233 (bubble viscometer method)(temperature: 25° C.±0.5° C., solvent: butyl carbitol, resin component(solid content) concentration: 40 mass %) of, for example, 0.22×10⁴ to2.00×10⁴ m²/s, preferably 0.3×10⁴ to 1.9×10⁴ m²/s, or more preferably0.4×10⁻⁴ to 1.8×10⁴ m²/s. The above-described kinetic viscosity can alsobe set to, for example, 0.22×10⁴ to 1.00×10⁴ m²/s, preferably 0.3×10⁴ to0.9×10⁴ m²/s, or more preferably 0.4×10⁻⁴ to 0.8×10⁻⁴ m²/s.

When the kinetic viscosity of the resin component exceeds theabove-described range, excellent flexibility and conformability toirregularities (described later) may not be given to the thermalconductive sheet. On the other hand, when the kinetic viscosity of theresin component is below the above-described range, boron nitrideparticles may not be oriented in a predetermined direction.

In the kinetic viscosity test in conformity with JIS K 7233 (bubbleviscometer method), the kinetic viscosity of the resin component ismeasured by comparing the bubble rising speed of a resin componentsample with the bubble rising speed of criterion samples (having a knownkinetic viscosity), and determining the kinetic viscosity of thecriterion sample having a matching rising speed to be the kineticviscosity of the resin component.

In the thermal conductive sheet, the proportion of the volume-basedboron nitride particle content (solid content, that is, the volumepercentage of boron nitride particles relative to a total volume of theresin component and the boron nitride particles) is, for example, 35 vol% or more, preferably 60 vol % or more, or more preferably 75 vol % ormore, and usually, for example, 95 vol % or less, or preferably 90 vol %or less.

When the proportion of the volume-based boron nitride particle contentis below the above-described range, the boron nitride particles may notbe oriented in a predetermined direction in the thermal conductivesheet. On the other hand, when the proportion of the volume-based boronnitride particle content exceeds the above-described range, the thermalconductive sheet may become fragile, and handleability andconformability to irregularities may be reduced.

The mixing ratio by mass of the boron nitride particles relative to 100parts by mass of the total amount of the components (boron nitrideparticles and resin component)(total solid content) forming the thermalconductive sheet is, for example, 40 to 95 parts by mass, or preferably65 to 90 parts by mass, and the mixing ratio by mass of the resincomponent relative to 100 parts by mass of the total amount of thecomponents forming the thermal conductive sheet is, for example, 5 to 60parts by mass, or preferably 10 to 35 parts by mass. The mixing ratio bymass of the boron nitride particles relative to 100 parts by mass of theresin component is, for example, 60 to 1900 parts by mass, or preferably185 to 900 parts by mass.

When two epoxy resins (a first epoxy resin and a second epoxy resin) areused in combination, the mass ratio (mass of the first epoxy resin/massof the second epoxy resin) of the first epoxy resin relative to thesecond epoxy resin can be set appropriately in accordance with thesoftening temperature and the like of the epoxy resins (the first epoxyresin and the second epoxy resin). For example, the mass ratio of thefirst epoxy resin relative to the second epoxy resin is 1/99 to 9911, orpreferably 10/90 to 90/10.

In the resin component, in addition to the above-described components(polymer), for example, a polymer precursor (for example, a lowmolecular weight polymer including oligomer), and/or a monomer arecontained.

FIG. 1 shows a perspective view of an embodiment of a thermal conductivesheet of the present invention, and FIG. 2 shows process drawings fordescribing a method for producing the thermal conductive sheet shown inFIG. 1.

Next, a method for producing a thermal conductive sheet as an embodimentof the present invention is described with reference to FIG. 1 and FIG.2.

In this method, first, the above-described components are blended at theabove-described mixing ratio and are stirred and mixed, therebypreparing a mixture.

In the stirring and mixing, in order to mix the components efficiently,for example, the solvent may be blended therein with the above-describedcomponents, or, for example, the resin component (preferably, thethermoplastic resin component) can be melted by heating.

Examples of the solvent include the above-described organic solvents.When the above-described curing agent and/or the curing accelerator areprepared as a solvent solution and/or a solvent dispersion liquid, thesolvent of the solvent solution and/or the solvent dispersion liquid canalso serve as a mixing solvent for the stirring and mixing withoutadding a solvent during the stirring and mixing. Alternatively, in thestirring and mixing, a solvent can be further added as a mixing solvent.

In the case when the stirring and mixing is performed using a solvent,the solvent is removed after the stirring and mixing.

To remove the solvent, for example, the mixture is allowed to stand atroom temperature for 1 to 48 hours; heated at 40 to 100° C. for 0.5 to 3hours; or heated under a reduced pressure atmosphere of, for example,0.001 to 50 kPa, at 20 to 60° C., for 0.5 to 3 hours.

When the resin component is to be melted by heating, the heatingtemperature is, for example, a temperature in the neighborhood of orexceeding the softening temperature of the resin component, to bespecific, 40 to 150° C., or preferably 70 to 140° C.

Next, in this method, the obtained mixture is hot-pressed.

To be specific, as shown in FIG. 2( a), as necessary, for example, themixture is hot-pressed with two releasing films 4 sandwiching themixture, thereby producing a pressed sheet 1A. Conditions for thehot-pressing are as follows: a temperature of, for example, 50 to 150°C., or preferably 60 to 140° C.; a pressure of, for example, 1 to 100MPa, or preferably 5 to 50 MPa; and a duration of, for example, 0.1 to100 minutes, or preferably 1 to 30 minutes.

More preferably, the mixture is hot-pressed under vacuum. The degree ofvacuum in the vacuum hot-pressing is, for example, 1 to 100 Pa, orpreferably 5 to 50 Pa, and the temperature, the pressure, and theduration are the same as those described above for the hot-pressing.

When the temperature, the pressure, and/or the duration in thehot-pressing is outside the above-described range, there is a case wherea porosity P (described later) of the thermal conductive sheet 1 cannotbe adjusted to a desired value.

The pressed sheet 1A obtained by the hot-pressing has a thickness of forexample, 50 to 1000 μm, or preferably 100 to 800 μm.

Next, in this method, as shown in FIG. 2( b), the pressed sheet 1A isdivided into a plurality of pieces (for example, four pieces), therebyproducing a divided sheet 1B (dividing step). In the division of thepressed sheet 1A, the pressed sheet 1A is cut along the thicknessdirection so that the pressed sheet 1A is divided into a plurality ofpieces when the pressed sheet 1A is projected in the thicknessdirection. The pressed sheet 1A is cut so that the respective dividedsheets 1B have the same shape when the divided sheets 1B are projectedin the thickness direction.

Next, in this method, as shown in FIG. 2( c), the respective dividedsheets 1B are laminated in the thickness direction, thereby producing alaminated sheet 1C (laminating step).

Thereafter, in this method, as shown in FIG. 2( a), the laminated sheet1C is hot-pressed (preferably hot-pressed under vacuum) (hot-pressingstep). The conditions for the hot-pressing are the same as theconditions for the hot-pressing of the above-described mixture.

The thickness of the hot-pressed laminated sheet 1C is, for example, 1mm or less, or preferably 0.8 mm or less, and usually is, for example,0.05 mm or more, or preferably 0.1 mm or more.

Thereafter, the series of the steps of the above-described dividing step(FIG. 2( b)), laminating step (FIG. 2( c)), and hot-pressing step (FIG.2( a)) are performed repeatedly, so as to allow boron nitride particles2 to be efficiently oriented in a predetermined direction in the resincomponent 3 in the thermal conductive sheet 1. The number of therepetition is not particularly limited, and can be set appropriatelyaccording to the charging state of the boron nitride particles. Thenumber of the repetition is, for example, 1 to 10 times, or preferably 2to 7 times.

In the above-described hot-pressing step (FIG. 2( a)), for example, aplurality of calendering rolls and the like can also be used for rollingthe mixture and the laminated sheet 1C.

The thermal conductive sheet 1 can be obtained in this manner.

The thickness of the obtained thermal conductive sheet 1 is, forexample, 1 mm or less, or preferably 0.8 mm or less, and usually, forexample, 0.05 mm or more, or preferably 0.1 mm or more.

In the thermal conductive sheet 1, the proportion of the volume-basedboron nitride particle content (solid content, that is, volumepercentage of boron nitride particles relative to the total volume ofthe resin component and the boron nitride particles is, as describedabove, for example, 35 vol % or more (preferably 60 vol % or more, ormore preferably 75 vol % or more), and usually 95 vol % or less(preferably 90 vol % or less).

When the proportion of the boron nitride particle content is below theabove-described range, the boron nitride particles 2 may not be orientedin a predetermined direction in the thermal conductive sheet 1.

When the resin component 3 is the thermosetting resin component, forexample, the series of the steps of the above-described dividing step(FIG. 2( b)), laminating step (FIG. 2( c)), and hot-pressing step (FIG.2( a)) are performed repeatedly for an uncured thermal conductive sheet1, thereby producing an uncured thermal conductive sheet 1 as is.

The thermal conductive sheet 1 is in semi-cured (in B-stage) state whenthermocompression bonded to the adherend to be described later, and thethermal conductive sheet 1 is cured by heat after the thermocompressionbonding to the adherend.

To cure the thermal conductive sheet 1 by heat, the above-describedhot-press or a dryer is used. Preferably, a dryer is used. Theconditions for the curing by heat are as follows: a temperature of, forexample, 60 to 250° C., or preferably 80 to 200° C.

In the thus obtained thermal conductive sheet 1, as shown in FIG. 1 andits partially enlarged schematic view, the longitudinal direction LD ofthe boron nitride particle 2 is oriented along a plane (surface)direction SD that crosses (is perpendicular to) the thickness directionTD of the thermal conductive sheet 1.

The calculated average of the angle formed between the longitudinaldirection LD of the boron nitride particle 2 and the plane direction SDof the thermal conductive sheet 1 (orientation angle a of the boronnitride particles 2 relative to the thermal conductive sheet 1) is, forexample, 25 degrees or less, or preferably 20 degrees or less, andusually 0 degree or more.

The orientation angle a of the boron nitride particle 2 relative to thethermal conductive sheet 1 is obtained as follows: the thermalconductive sheet 1 is cut along the thickness direction with a crosssection polisher (CP); the cross section thus appeared is photographedwith a scanning electron microscope (SEM) at a magnification thatenables observation of 200 or more boron nitride particles 2 in thefield of view; a tilt angle α between the longitudinal direction LD ofthe boron nitride particle 2 and the plane direction SD (directionperpendicular to the thickness direction TD) of the thermal conductivesheet 1 is obtained from the obtained SEM photograph; and the averagevalue of the tilt angles α is calculated.

The thus obtained thermal conductivity in the plane direction SD of thethermal conductive sheet 1 is 4 W/m·K or more, preferably 5 Wm·K ormore, more preferably 10 W/m·K or more, even more preferably 15 W/m·K ormore, or particularly preferably 25 W/m·K or more, and usually 200 W/m·Kor less.

The thermal conductivity in the plane direction SD of the thermalconductive sheet 1 is substantially the same before and after the curingby heat when the resin component 3 is the thermosetting resin component.

When the thermal conductivity in the plane direction SD of the thermalconductive sheet 1 is below the above-described range, thermalconductivity in the plane direction SD is insufficient, and thereforethere is a case where the thermal conductive sheet 1 cannot be used forheat dissipation that requires thermal conductivity in such a planedirection SD.

The thermal conductivity in the plane direction SD of the thermalconductive sheet 1 is measured by a pulse heating method. In the pulseheating method, the xenonflash analyzer “LFA-447” (manufactured by ErichNETZSCH GmbH & Co. Holding KG) is used.

The thermal conductivity in the thickness direction TD of the thermalconductive sheet 1 is, for example, 0.5 to 15 W/m·K, or preferably 1 to10 W/m·K.

The thermal conductivity in the thickness direction TD of the thermalconductive sheet 1 is measured by a pulse heating method, a laser flashmethod, or a TWA method. In the pulse heating method, theabove-described device is used, in the laser flash method, “TC-9000”(manufactured by Ulvac, Inc.) is used, and in the TWA method, “ai-Phasemobile” (manufactured by ai-Phase Co., Ltd) is used.

Thus, the ratio of the thermal conductivity in the plane direction SD ofthe thermal conductive sheet 1 relative to the thermal conductivity inthe thickness direction TD of the thermal conductive sheet 1 (thermalconductivity in the plane direction SD/thermal conductivity in thethickness direction TD) is, for example, 1.5 or more, preferably 3 ormore, or more preferably 4 or more, and usually 20 or less.

Although not shown in FIG. 1, for example, pores (gaps) are formed inthe thermal conductive sheet 1.

The proportion of the pores in the thermal conductive sheet 1, that is,a porosity P, can he adjusted by setting the proportion of the boronnitride particle 2 content (volume-based), and further setting thetemperature, the pressure, and/or the duration at the time of hotpressing the mixture of the boron nitride particle 2 and the resincomponent 3 (FIG. 2( a)). To be specific, the porosity P can be adjustedby setting the temperature, the pressure, and/or the duration of the hotpressing (FIG. 2( a)) within the above-described range.

The porosity P of the thermal conductive sheet 1 is, for example, 30 vol% or less, or preferably 10 vol % or less.

The porosity P is measured by, for example, as follows: the thermalconductive sheet 1 is cut along the thickness direction with a crosssection polisher (CP); the cross section thus appeared is observed witha scanning electron microscope (SEM) at a magnification of 200 to obtainan image; the obtained image is binarized based on the pore portion andthe non-pore portion; and the area ratio, i.e., the ratio of the poreportion area to the total area of the cross section of the thermalconductive sheet 1 is determined by calculation.

The thermal conductive sheet 1 has a porosity P2 after curing of,relative to a porosity P1 before curing, for example, 100% or less, orpreferably 50% or less.

For the measurement of the porosity P (P1), when the resin component 3is a thermosetting resin component, the thermal conductive sheet 1before being cured by heat is used.

When the porosity P of the thermal conductive sheet 1 is within theabove-described range, the conformability to irregularities (describedlater) of the thermal conductive sheet 1 can be improved.

Meanwhile, the thermal conductive sheet 1 does not fall off from theadherend in the initial adhesion test (1) below. That is, thetemporarily fixed state between the thermal conductive sheet 1 and theadherend is kept.

Initial Adhesion Test (1): The thermal conductive sheet 1 isthermocompression bonded on top of an adherend that is placed along ahorizontal direction to be temporarily fixed thereon, allowed to standfor 10 minutes, and the adherend is turned over to be upside down.

Examples of the adherend include a substrate made of stainless steel(e.g., SUS 304 and the like), or a mounting substrate for notebook PC onwhich a plurality of electronic components such as IC (integratedcircuit) chips, condensers, coils, and resistors are mounted. In themounting substrate for notebook PC, the electronic components areusually disposed on the top face (one side) with a space providedtherebetween in the plane direction (the plane direction of the mountingsubstrate for notebook PC).

In pressure bonding, for example, while a sponge roll made of a resinsuch as silicone resin is pressed against the thermal conductive sheet1, the sponge roll is rolled on the surface of the thermal conductivesheet 1.

The temperature of the thermocompression bonding is, when the resincomponent 3 is a thermosetting resin component (e.g., epoxy resin), forexample, 80° C.

On the other hand, when the resin component 3 is a thermoplastic resincomponent (e.g., polyethylene), the temperature of the thermocompressionbonding is a temperature higher by 10 to 30° C. than the softening pointor the melting point of the thermoplastic resin component; preferably atemperature higher by 15 to 25° C. than the softening point or themelting point of the thermoplastic resin component; more preferably, atemperature higher by 20° C. than the softening point or the meltingpoint of the thermoplastic resin component; or to be specific, atemperature of 120° C. (that is, the softening point or the meltingpoint of the thermoplastic resin component is 100° C., and thetemperature higher by 20° C. than 100° C. is 120° C.).

When the thermal conductive sheet 1 falls off from the adherend in theabove-described initial adhesion test (1), that is, when the temporarilyfixed state between the thermal conductive sheet 1 and the adherend isnot kept, the thermal conductive sheet 1 cannot be reliably temporarilyfixed to the adherend.

When the resin component 3 is a thermosetting resin component, thethermal conductive sheet 1 to be tested in the initial adhesion test (1)and the initial adhesion test (2) (described later) is a thermalconductive sheet 1 before being cured, and the thermal conductive sheet1 will be in B-stage based on the thermocompression bonding in theinitial adhesion test (1) and the initial adhesion test (2).

When the resin component 3 is a thermoplastic resin component, thethermal conductive sheet 1 to be tested in the initial adhesion test (1)and the initial adhesion test (2) (described later) is a solid thermalconductive sheet 1, and the thermal conductive sheet 1 will be softenedbased on the thermocompression bonding in the initial adhesion test (1)and the initial adhesion test (2).

Preferably, the thermal conductive sheet 1 does not fall off from theadherend in both of the above-described initial adhesion test (1) andthe initial adhesion test (2) described below. That is, the temporarilyfixed state between the thermal conductive sheet 1 and the adherend iskept.

Initial Adhesion Test (2): The thermal conductive sheet 1 isthermocompression bonded on top of an adherend that is placed along ahorizontal direction to be temporarily fixed thereon, and then allowedto stand for 10 minutes, and thereafter, the adherend is disposed alonga vertical direction (up-down directions).

The temperature in the thermocompression bonding of the Initial AdhesionTest (2) is the same as the temperature of thermocompression bonding inthe Initial Adhesion Test (1).

When the thermal conductive sheet 1 is evaluated in the bend test inconformity with the cylindrical mandrel method of JIS K 5600-5-1 underthe test conditions shown below, preferably, no fracture is observed.

Test Conditions:

Test Device: Type 1

Mandrel: diameter 10 mm

Bending Angle: 90 degrees or more

Thickness of the thermal conductive sheet 1: 0.3 mm

FIGS. 3 and 4 show perspective views of the Type I test device. In thefollowing, the Type I test device is described.

In FIGS. 3 and 4, a Type I test device 10 includes a first flat plate11; a second flat plate 12 disposed in parallel with the first flatplate 11; and a mandrel (rotation axis) 13 provided for allowing thefirst flat plate 11 and the second flat plate 12 to rotate relatively.

The first flat plate 11 is formed into a generally rectangular flatplate. A stopper 14 is provided at one end portion (free end portion) ofthe first flat plate 11. The stopper 14 is formed on the surface of thesecond flat plate 12 so as to extend along the one end portion of thesecond flat plate 12.

The second flat plate 12 is formed into a generally rectangular flatplate, and one side thereof is disposed so as to be adjacent to one side(the other end portion (proximal end portion) that is opposite to theone end portion where the stopper 14 is provided) of the first flatplate 11.

The mandrel 13 is formed so as to extend along one side of the firstflat plate 11 and one side of the second flat plate 12 that are adjacentto each other.

In the Type I test device 10, as shown in FIG. 3, the surface of thefirst flat plate 11 is flush with the surface of the second flat plate12 before the start of the bend test.

To perform the bend test, the thermal conductive sheet 1 is placed onthe surface of the first flat plate 11 and the surface of the secondflat plate 12. The thermal conductive sheet 1 is placed so that one sideof the thermal conductive sheet 1 is in contact with the stopper 14.

Then, as shown in FIG. 4, the first flat plate 11 and the second flatplate 12 are rotated relatively. In particular, the free end portion ofthe first flat plate 11 and the free end portion of the second flatplate 12 are rotated to a predetermined angle with the mandrel 13 as thecenter. To he specific, the first flat plate 11 and the second flatplate 12 are rotated so as to bring the surface of the free end portionsthereof closer (oppose each other).

In this way, the thermal conductive sheet 1 is bent with the mandrel 13as the center, conforming to the rotation of the first flat plate 11 andthe second flat plate 12.

More preferably, no fracture is observed in the thermal conductive sheet1 even when the bending angle is set to 180 degrees under theabove-described test conditions.

When the resin component 3 is the thermosetting resin component, asemi-cured (in B-stage) thermal conductive sheet 1 (that is, the thermalconductive sheet 1 before being cured by heat) is tested in the bendtest.

When the fracture is observed in the bend test at the above bendingangle in the thermal conductive sheet 1, there is a case where excellentflexibility and conformability to irregularities (described later)cannot be given to the thermal conductive sheet 1.

In the thermal conductive sheet 1, the maximum cutting resistance on thecutting blade at the time of cutting is 120 N/30 mm or less.

When the maximum cutting resistance exceeds the above upper limit,brittleness of the thermal conductive sheet 1 increases, and problemssuch as chipping and fracture at the time of processing are caused.

For the method for measuring the maximum cutting resistance, the methoddescribed in Japanese Unexamined Patent Publication No. 2003-181967 isused.

To be specific, first, the thermal conductive sheet 1 (length 30mm×width 50 mm) is die cut with a cutting blade (Thomson blade: MIR(manufactured by Nakayama Co., Ltd., blade angle: 43°, blade length: 40mm, blade thickness: 0.71 mm, blade height: 23.6 mm) attached to ahydraulic press at a stroke of 30 SPM (stroke per minute) up to 70 μmrelative to the faceplate (so that the blade edge enters (reaches) up tothe depth of 70 μm from the top face of the faceplate that supports thebottom face of the thermal conductive sheet 1 to the lower side(inwardly)), and the cutting resistance applied on the faceplate ismeasured using a load cell (time resolution: 500 Hz, load resolution: 9mN, maximum measurement load: 3.6 kN, load capacity: 27 kN) and acomputer at 23° C.

When the resin component 3 in the thermal conductive sheet 1 is athermosetting resin component, the maximum cutting resistance is themaximum cutting resistance of a semi-cured (in B-stage) thermalconductive sheet 1 (that is, a thermal conductive sheet 1 before beingcured by heat).

Furthermore, for example, when the thermal conductive sheet 1 isevaluated in the 3-point bending test in conformity with JIS K 7171(2008) under the test conditions shown below, no fracture is observed.

Test Conditions:

Test piece: size 20 mm×15 mm

Distance between supporting points: 5 mm

Testing speed: 20 mm/min (indenter depressing speed)

Bending angle: 120 degrees

Evaluation method: Presence or absence of fracture such as cracks at thecenter of the test piece is observed visually when tested under theabove-described test conditions.

In the 3-point bending test, when the resin component 3 is athermosetting resin component, the thermal conductive sheet 1 beforebeing cured by heat is used.

Therefore, the thermal conductive sheet 1 is excellent in conformabilityto irregularities because no fracture is observed in the above-described3-point bending test. The conformability to irregularities is, when thethermal conductive sheet 1 is provided on an object with irregularities(for example, on the above-described mounting substrate for notebookPC), a property of the thermal conductive sheet 1 that conforms to be inclose contact with the irregularities (for example, irregularitiesformed by the above-described electronic components).

A mark such as, for example, letters and symbols can be given to thethermal conductive sheet 1. That is, the thermal conductive sheet 1 isexcellent in mark adhesion. The mark adhesion is a property of thethermal conductive sheet 1 that allows reliable adhesion of theabove-described mark thereon.

The mark can be adhered (applied, fixed, or firmly fixed) to the thermalconductive sheet 1, to be specific, by printing, engraving, or the like.

Examples of printing include, for example, inkjet printing, reliefprinting, intaglio printing, and laser printing.

When the mark is to be printed by inkjet printing, relief printing, orintaglio printing, for example, an ink fixing layer for improving mark'sfixed state can be provided on the surface (printing side) of thethermal conductive sheet 1.

When the mark is to be printed by laser printing, for example, a tonerfixing layer for improving marks fixed state can be provided on thesurface (printing side) of the thermal conductive sheet 1.

Examples of engraving include laser engraving and punching.

Furthermore, the above-described thermal conductive sheet 1 is excellentin thermal conductivity in the plane direction SD, and at the same time,excellent in adhesion (initial adhesion) to an adherend afterthermocompression bonding at a predetermined temperature.

Thus, by thermocompression bonding the thermal conductive sheet 1 to theadherend, the thermal conductive sheet 1 can be reliably fixed(temporarily fixed) to the adherend.

Thus, by temporarily fixing the thermal conductive sheet 1 in B-stage tothe adherend, and thereafter, curing the thermal conductive sheet 1 byheating, the thermal conductive sheet I can be reliably adhered to theadherend, and the thermal conductive sheet 1 allows the heat from theadherend to be conducted efficiently along the plane direction SD of thethermal conductive sheet 1.

The adherend is not particularly limited, and examples thereof alsoinclude, in addition to the above-described electronic components,light-emitting diodes.

On the other hand, there is a case where it is desired to remove oncethe thermal conductive sheet 1 for position adjustment as necessaryafter temporarily fixing the thermal conductive sheet 1 to the adherend,and bonded thereto again (reworking). In this case, the above-describedthermal conductive sheet 1 is in B-stage, and reworkability isexcellent. Thus, residue of the thermal conductive sheet 1 on thesurface of the adherend can be prevented at the time of removing, and atthe same time, reworking of the thermal conductive sheet 1 is easy.

Furthermore, even if the thermal conductive sheet 1 remained on thesurface of the adherend, because the thermal conductive sheet 1 isuncured (before being cured), the residue can be easily wiped off(removed).

EXAMPLES

Hereinafter, the present invention is described in further detail withreference to Examples. However, the present invention is not limited tothose described in Examples.

Example 1

The components described below were blended, stirred, and allowed tostand at room temperature (23° C.) for one night, thereby allowingmethyl ethyl ketone (dispersion medium for the curing agent) tovolatilize and preparing a semi-solid mixture. The details of thecomponents were as follows: 13.42 g of PT-110 (trade name, plate-likeboron nitride particles, average particle size (light scattering method)45 μm, manufactured by Momentive Performance Materials Inc.), 1.0 g ofjER®828 (trade name, bisphenol A epoxy resin, first epoxy resin, liquid,epoxy equivalent 184 to 194 g/eqiv., softening temperature (ring andball method) below 25° C., melt viscosity (80° C.) 70 mPa·s,manufactured by Japan Epoxy Resins Co., Ltd.), 2.0 g of EPPN-501HY(trade name, triphenylmethane epoxy resin, second epoxy resin, solid,epoxy equivalent 163 to 175 g/eqiv., softening temperature (ring andball method) 57 to 63° C., manufactured by NIPPON KAYAKU Co., Ltd), and3 g (solid content 0.15 g) (5 mass% per total amount of epoxy resins ofjER®828 and EPPN-501HY) of a curing agent (a dispersion liquid of 5mass% Curezol® 2P4MHZ-PW (trade name, manufactured by Shikoku ChemicalsCorporation.) in methyl ethyl ketone).

In the above-described blending, the volume percentage (vol %) of theboron nitride particles relative to the total volume of the solidcontent excluding the curing agent (that is, solid content of the boronnitride particle and epoxy resin) was 70 vol %.

Then, the obtained mixture was sandwiched by two silicone-treatedreleasing films, and then these were hot-pressed with a vacuum hot-pressat 80° C. under an atmosphere (vacuum atmosphere) of 10 Pa with a loadof 5 tons (20 MPa) for 2 minutes. A pressed sheet having a thickness of0.3 min was thus obtained (ref: FIG. 2( a)).

Thereafter, the obtained pressed sheet was cut so as to be divided intoa plurality of pieces when projected in the thickness direction of thepressed sheet. Divided sheets were thus obtained (ref: FIG. 2( b)).Next, the divided sheets were laminated in the thickness direction, Alaminated sheet was thus obtained (rcf: FIG. 2( c)).

Then, the obtained laminated sheet was hot-pressed under the sameconditions as described above with the above-described vacuum hot-press(ref: FIG. 2( a)).

Then, a series of the above-described operations of cutting, laminating,and hot-pressing (ref FIG. 2) was repeated four times. A thermalconductive sheet (in uncured state) having a thickness of 0.3 mm wasthus obtained.

Examples 2 to 16

A thermal conductive sheet was obtained in the same manner as in Example1 in accordance with the mixing formulation and production conditions ofTables 1 to 3.

Evaluation 1. Thermal Conductivity

The thermal conductivity of the thermal conductive sheets obtained inExamples 1 to 16 was measured.

That is, the thermal conductivity in the plane direction (SD) wasmeasured by a pulse heating method using a xenon flash analyzer“LFA-447” (manufactured by Erich NETZSCH GmbH & Co. Holding KG).

The results are shown in Tables 1 to 3.

2. Initial Adhesion Test 2-1. Initial Adhesion Test to MountingSubstrate for Notebook PC

Initial adhesion tests (1) and (2) were performed for adhesion of thethermal conductive sheets obtained in Examples 1 to 16 to a mountingsubstrate for notebook PC. On the mounting substrate, a plurality ofelectronic components are mounted.

That is, the thermal conductive sheet was temporarily fixed to thesurface (the side on which the electronic components are mounted) alongthe horizontal direction of the mounting substrate for notebook PC usinga sponge roll made of silicone resin by thermocompression bonding at 80°C. (Examples 1 to 9 and Examples 11 to 16) or 120° C. (Example 10), andthen allowed to stand for 10 minutes, and thereafter, the mountingsubstrate for notebook PC was disposed along the up-down directions(Initial Adhesion Test (2)).

Afterwards, the mounting substrate for notebook PC was positioned sothat the thermal conductive sheet faces downward (that is, turned overto be upside down from the position of the temporarily fixing) (InitialAdhesion Test (1)).

Then, in the above-described Initial Adhesion Test (1) and Initial.Adhesion Test (2), the thermal conductive sheet was evaluated based onthe criteria below. The results are shown in Tables 1 to 3.

<Criteria>

Good: It was confirmed that the thermal conductive sheet did not falloff from the mounting substrate for notebook PC.

Bad: It was confirmed that the thermal conductive sheet fell off fromthe mounting substrate for notebook PC.

2-2. Initial Adhesion Test to Stainless Steel Substrate

Initial adhesion tests (1) and (2) were conducted in the same manner asdescribed above for adhesion of the thermal conductive sheets obtainedin Examples 1 to 16 to a stainless steel substrate (made of SUS 304).

As a result, the thermal conductive sheets did not fall off from thestainless steel substrate in both of the initial adhesion tests (1) and(2).

Then, in the above-described Initial Adhesion Test (1) and InitialAdhesion Test (2), the thermal conductive sheet was evaluated based onthe criteria below. The results are shown in Tables 1 to 3.

<Criteria>

Good: It was confirmed that the thermal conductive sheet did not falloff from the stainless steel substrate.

Bad: It was confirmed that the thermal conductive sheet fell off fromthe stainless steel substrate.

3. Maximum Cutting Resistance

The maximum cutting resistance on the cutting blade at the time ofcutting the thermal conductive sheets obtained in Examples 1 to 16 wasmeasured in accordance with the method described in Japanese UnexaminedPatent Publication No.2003-181967.

That is, the thermal conductive sheet (length 30 mm×width 50 mm) was diecut with a cutting blade (Thomson blade: MIR (manufactured by NakayamaCo., Ltd., blade angle: 43°, blade length: 40 mm, blade thickness: 0.71mm, blade height: 23.6 mm) attached to a hydraulic press at a stroke of30 SPM (stroke per minute), and up to 70 μm relative to the faceplate(so that the blade edge enters (reaches) up to the depth of 70 μm fromthe top face of the faceplate that supports the bottom face of thethermal conductive sheet to the lower side (inwardly)), and the cuttingresistance applied on the faceplate was measured using a load cell (timeresolution: 500 Hz, load resolution: 9 mN, maximum measurement load: 3.6kN, load capacity: 27 kN) and a computer at 23° C.

As a result, it was confirmed that the maximum cutting resistance on thecutting blade was 120 N/30 mm or less in any of the thermal conductivesheets obtained in Examples 1 to 16.

4. Workability in Die Cutting

Five types of Thomson die cut blades (manufactured by NISHIYAMASEISAKUSHO CO., LTD.) shown below were prepared.

-   (1) Rhombus (⋄): 10 ram square-   (2) Circular (◯): Diameter 23 mm-   (3) Circular (◯): Diameter 17 mm-   (4) Number Sign Shaped Rhombus (#): 6 mm square (3 columns×3 rows=9)-   (5) Number Sign Shaped Rhombus (#): 8 mm square (9 columns×9    rows=81)

Workability Test A

The thermal conductive sheets obtained in Examples 1 to 16 on areleasing film (polyethylene terephthalate film treated for release)were placed on rubber sheet, and were die cut using the above-describedThomson blades together with the releasing film as the support.

As a result, no defect was caused in the thermal conductive sheets, andof the thermal conductive sheets obtained in Examples 1 to 16 and any ofthe blades used, no thermal conductive sheets stuck on the blades.

After picking up the releasing film side by sucking andthermocompression bonding the thermal conductive sheet side on themounting substrate, the releasing film was released and removed. At thistime as well, no chipping or crumbling was caused in any of the thermalconductive sheets obtained in Examples 1 to 16.

Workability Test B

The thermal conductive sheets obtained in Examples 1 to 16 on areleasing film (polyethylene terephthalate film treated for release)were disposed on polystyrene sheet (Nippla sheet (product name),thickness: 0.3 mm, manufactured by Nihon Plastic), and allowed to behalf-cut using the above-described Thomson die cut blades from thepolystyrene sheet side so as not to cut the releasing film.

As a result, no defect was caused in the thermal conductive sheets, andof the thermal conductive sheets obtained in Examples 1 to 16 and any ofthe blades used, no thermal conductive sheets stuck on the blades.

In addition, because the releasing film was not cut, the sheet (thelaminate of the releasing film that was not being die cut and thethermal conductive sheet thereon that was being die cut) was used as isand thermocompression bonded on the mounting substrate with the thermalconductive sheet side facing the mounting substrate, and thereafter, thereleasing film was released and removed. At this time, in any of thethermal conductive sheets obtained in Examples 1 to 16, there was noproblem of re-adhesion of cut pieces at the cutting end faces of thethermal conductive sheet, and transferring was easily performed.

5. Porosity (P)

The porosity (P1) of the thermal conductive sheets before being cured byheat in Examples 1 to 16 was measured by the following measurementmethod.

Measurement method of porosity: The thermal conductive sheet was cutalong the thickness direction with a cross section polisher (CP); andthe cross section thus appeared was observed with a scanning electronmicroscope (SEM) at a magnification of 200. The obtained image wasbinarized based on the pore portion and the non-pore portion; and thearea ratio, i.e., the ratio of the pore portion area to the total areaof the cross section of the thermal conductive sheet was calculated.

The results are shown in Tables 1 to 3.

6. Conformability to Irregularities (3-Point Bending Test)

The 3-point bending test in conformity with JIS K 7171 (2010) wascarried out for the thermal conductive sheets obtained in Examples 1 to16 with the following test conditions, thus evaluating conformability toirregularities with the following evaluation criteria. The results areshown in Tables 1 to 3.

Test Conditions:

Test piece: size 20 mm×15 mm

Distance between supporting points: 5 mm

Testing speed: 20 mm/min (indenter depressing speed)

Bending angle: 120 degrees

(Evaluation Criteria)

Excellent: No fracture was observed.

Good: Almost no fracture was observed.

Bad: Fracture was clearly observed.

7. Printed Mark Visibility (Mark Adhesion by Printing: Mark Adhesion byInkjet Printing or Laser Printing)

Marks were printed on the thermal conductive sheets of Examples 1 to 16by inkjet printing and laser printing, and the mark was observed.

As a result, it was confirmed that the mark was excellently visible inboth cases of inkjet printing and laser printing, and that mark adhesionby printing was excellent in any of the thermal conductive sheets ofExamples 1 to 16.

TABLE 1 Average Particle Size Example (μm) Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5Ex. 6 Mixing Boron Nitride PT-110*¹ 45 13.42 3.83 5.75 12.22 23 —Formulation Particle/g*^(A)/ [70] [40]   [50]   [68]   [80]   of [vol%]*^(B)/ [69] [38.8] [48.8] [66.9] [79.2] Components [vol %]*^(C)UHP-1*²  9 — — — — — 12.22 [68]   [66.9] Polymer Thermosetting EpoxyResin Epoxy Resin A*³ — 3 3 3 3 3 Resin Compositon (Semi-solid) EpoxyResin B*⁴ 1 — — — — — (Liquid) Epoxy Resin C*⁵ — — — — — — (Solid) EpoxyResin D*⁶ 2 — — — — — (Solid) Curing Agent*⁷ — 3 3 3 3 3 (Solid Contentin Grams) (0.15) (0.15) (0.15) (0.15) (0.15) Curing Agent*⁸ 3 — — — — —(Solid Content in Grams) (0.15) Thermoplastic Polyethylene*⁹ — — — — — —Resin Production Hot- Temperature (° C.) 80 80 80 80 80 80 Conditionspressing Number of Time (times)*^(D) 5 5 5 5 5 5 Load (MPa)/(tons) 20/520/5 20/5 20/5 20/5 20/5 Evaluation Thermal Thermal Conductivity PlaneDirection (SD) 30 4.5 6.0 30.0 32.5 17.0 Conductive (W/m · K) ThicknessDirection (TD) 2.0 1.3 3.3 5.0 5.5 5.8 Sheet Ratio (SD/TD) 15.0 3.5 1.86.0 5.9 2.9 Initial Adhesion Test To Mounting Test Good Good Good GoodGood Good Substrate for (1) Notebook PC Test Good Good Good Good GoodGood (2) To Stainless Test Good Good Good Good Good Good Steel Substrate(1) Test Good Good Good Good Good Good (2) Maximum Cutting Resistance(N/30 mm) Good Good Good Good Good Good Workability in Die Cutting TestA Good Good Good Good Good Good Test B Good Good Good Good Good GoodPorosity (vol %) 4 0 0 5 12 6 Conformability to Irregularities/3-pointbending test Excellent Good Good Good Good Good JIS K 7171 (2008) BoronNitride Orientation Angle (α)(degrees) 12 18 18 15 13 20 Particleg*^(A): Blended Weight [vol %]*^(B): Percentage relative to the totalvolume of the thermal conductive sheet (excluding curing agent) [vol%]*^(C): Percentage relative to the total volume of the thermalconductive sheet Number of Time*^(D): Number of times of hot-pressing oflaminated sheet

TABLE 2 Average Particle Size Example (μm) Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex.11 Mixing Boron Nitride PT-110*¹ 45 12.22 12.22 12.22 3.83 13.42Formulation Particle/g*^(A)/ [68]   [68]   [68]   [60] [70] of [vol%]*^(B)/ [66.9] [66.9] [66.9] [60] [69] Components [vol %]*^(C) UHP-1*² 9 — — — — — Polymer Thermosetting Epoxy Resin Epoxy Resin A*³ — — — — —Resin Compositon (Semi-solid) Epoxy Resin B*⁴ 1.5 3 — — — (Liquid) EpoxyResin C*⁵ 1.5 — 3 — — (Solid) Epoxy Resin D*⁶ — — — — 3 (Solid) CuringAgent*⁷ 3 3 3 — 3 (Solid Content in Grams) (0.15) (0.15) (0.15) (0.15)Curing Agent*⁸ — — — — — (Solid Content in Grams) ThermoplasticPolyethylene*⁹ — — — 1 — Resin Production Hot- Temperature (° C.) 80 8080 120 80 Conditions pressing Number of Time (times)*^(D) 5 5 5 5 5 Load(MPa)/(tons) 20/5 20/5 20/5 4/1 20/5 Evaluation Thermal ThermalConductivity Plane Direction (SD) 30.0 30.0 30.0 20 24.5 Conductive (W/m· K) Thickness Direction (TD) 5.0 5.0 5.0 2.0 2.1 Sheet Ratio (SD/TD)6.0 6.0 6.0 10.0 11.7 Initial Adhesion Test To Mounting Test Good GoodGood Good Good Substrate for (1) Notebook PC Test Good Good Good GoodGood (2) To Stainless Test Good Good Good Good Good Steel Substrate (1)Test Good Good Good Good Good (2) Maximum Cutting Resistance (N/30 mm)Good Good Good Good Good Workability in Die Cutting Test A Good GoodGood Good Good Test B Good Good Good Good Good Porosity (vol %) 4 2 13 110 Conformability to Irregularities/3-point bending test Good Good BadBad Bad JIS K 7171 (2008) Boron Nitride Orientation Angle (α)(degrees)15 16 16 15 16 Particle g*^(A): Blended Weight [vol %]*^(B): Percentagerelative to the total volume of the thermal conductive sheet (excludingcuring agent) [vol %]*^(C): Percentage relative to the total volume ofthe thermal conductive sheet Number of Time*^(D): Number of times ofhot-pressing of laminated sheet

TABLE 3 Average Particle Size Example (μm) Ex. 12 Ex. 13 Ex. 14 Ex. 15Ex. 16 Mixing Boron Nitride PT-110*¹ 45 3.83 13.42 13.42 13.42 13.42Formulation Particle/g*^(A)/ [40]   [70] [70] [70] [70] of [vol %]*^(B)/[37.7] [69] [69] [69] [69] Components [vol %]*^(C) UHP-1*²  9 — — — — —Polymer Thermosetting Epoxy Resin Epoxy Resin A*³ 3 3 3 3 3 ResinCompositon (Semi-solid) Epoxy Resin B*⁴ — — — — — (Liquid) Epoxy ResinC*⁵ — — — — — (Solid) Epoxy Resin D*⁶ — — — — — (Solid) Curing Agent*⁷ 63 3 3 3 (Solid Content in Grams) (0.3) (0.15) (0.15) (0.15) (0.15)Curing Agent*⁸ — — — — — (Solid Content in Grams) ThermoplasticPolyethylene*⁹ — — — — — Resin Production Hot- Temperature (° C.) 80 6070 80 80 Conditions pressing Number of Time (times)*^(D) 5 5 5 5 5 Load(MPa)/(tons) 20/5 20/5 20/5 20/5 40/10 Evaluation Thermal ThermalConductivity Plane Direction (SD) 4.1 10.5 11.2 32.5 50.7 Conductive(W/m · K) Thickness Direction (TD) 1.1 2.2 3.0 5.5 7.3 Sheet Ratio(SD/TD) 3.7 4.8 3.7 5.9 6.9 Initial Adhesion Test To Mounting Test GoodGood Good Good Good Substrate for (1) Notebook PC Test Good Good GoodGood Good (2) To Stainless Test Good Good Good Good Good Steel Substrate(1) Test Good Good Good Good Good (2) Maximum Cutting Resistance (N/30mm) Good Good Good Good Good Workability in Die Cutting Test A Good GoodGood Good Good Test B Good Good Good Good Good Porosity (vol %) 0 29 268 3 Conformability to Irregularities/3-point bending test ExcellentExcellent Excellent Excellent Good JIS K 7171 (2008) Boron NitrideOrientation Angle (α)(degrees) 20 17 15 15 13 Particle g*^(A): BlendedWeight [vol %]*^(B): Percentage relative to the total volume of thethermal conductive sheet (excluding curing agent) [vol %]*^(C):Percentage relative to the total volume of the thermal conductive sheetNumber of Time*^(D): Number of times of hot-pressing of laminated sheet

In Tables 1 to 3, values for the components are in grams unlessotherwise specified.

In the rows of “boron nitride particles” in Tables 1 to 3, values on thetop represent the Blended Weight (g) of the boron nitride particles;values in the middle represent the volume percentage (vol %) of theboron nitride particles relative to the total volume of the solidcontent excluding the curing agent in the thermal conductive sheet (thatis, solid content of the boron nitride particles, and epoxy resin orpolyethylene); and values at the bottom represent the volume percentage(vol %) of the boron nitride particles relative to the total volume ofthe solid content in the thermal conductive sheet (that is, solidcontent of boron nitride particles, epoxy resin, and curing agent).

For the components with “*” added in Tables 1 to 3, details are givenbelow.

-   PT-110*¹: trade name, plate-like boron nitride particles, average    particle size (light scattering method) 45 μm, manufactured by    Momentive Performance Materials Inc.-   IP-1*²: trade name: SHOBN®UHP-1, plate-like boron nitride particles,    average particle size (light scattering method) 9 μm, manufactured    by Showa Denko K.K.-   Epoxy Resin A*³: OGSOL EG (trade name), bisaryifluorene epoxy resin,    semi-solid, epoxy equivalent 294 g/eqiv., softening temperature    (ring and ball test) 47° C., melt viscosity (80° C.) 1360 mPa·s,    manufactured by Osaka Gas Chemicals Co., Ltd.-   Epoxy Resin B*⁴: jER® 828 (trade name), bisphenol A epoxy resin,    liquid, epoxy equivalent 184 to 194 g/eqiv., softening temperature    (ring and ball test) below 25° C., melt viscosity (80° C.) 70 mPa·s,    manufactured by Japan Epoxy Resins Co., Ltd.-   Epoxy Resin C*⁵: jER® 1002 (trade name), bisphenol A epoxy resin,    solid, epoxy equivalent 600 to 700 g/eqiv., softening temperature    (ring and ball test) 78° C., melt viscosity (80° C.) 10000 mPa·s or    more (measurement limit or more), manufactured by Japan Epoxy Resins    Co., Ltd.-   Epoxy Resin D*⁶: EPPN-501HY (trade name), triphenylmethane epoxy    resin, solid, epoxy equivalent 163 to 175 g/eqiv., softening    temperature (ring and ball test) 57 to 63° C., manufactured by    NIPPON KAYAKU Co., Ltd.-   Curing Agent*⁷: a solution of 5 mass % Curezol® 2PZ (trade name,    manufactured by Shikoku Chemicals Corporation) in methyl ethyl    ketone.-   Curing Agent”: a dispersion of 5 mass % Curezol® 2P4MHZ-PW (trade    name, manufactured by Shikoku Chemicals Corporation) in methyl ethyl    ketone.-   Polyethylene*⁹: low density polyethylene, weight average molecular    weight (Mw) 4000, number average molecular weight (Mn) 1700, melting    point 100° C. to 105° C., manufactured by Sigma-Aldrich Co.

While the illustrative embodiments of the present invention are providedin the above description, such is for illustrative purpose only and itis not to be construed as limiting the scope of the present invention.Modification and variation of the present invention that will be obviousto those skilled in the art is to be covered by the following claims.

1. A thermal conductive sheet comprising a plate-like boron nitrideparticle, wherein the thermal conductivity in a direction perpendicularto the thickness direction of the thermal conductive sheet is 4 W/m·K ormore, and the thermal conductive sheet does not fall off from anadherend in the initial adhesion test (1) below: Initial Adhesion Test(1): The thermal conductive sheet is temporarily fixed on an adherendalong a horizontal direction, and thereafter the adherend is turned overto be upside down.
 2. A thermal conductive sheet comprising a plate-likeboron nitride particle, wherein the thermal conductivity in a directionperpendicular to the thickness direction of the thermal conductive sheetis 4 W/m·K or more, and the maximum cutting resistance on a cuttingblade at the time of cutting the thermal conductive sheet is 120 N/30 mmor less.